From Drizzle To Sizzle

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Those clouds that were around Thursday morning, if images of clouds aren't burned into your memory yet, get a good look at them, because it's the last we'll see of them for a few days.

As you might have heard (or read), it's going to get quite hot around here. Highs Friday are expected to be in the mid 80s to low 90s, and again on Saturday before we begin to cool off a bit of Sunday.

You'll probably hear a lot about "thermal troughs" or "heat lows" and "marine pushes" over the next couple of days during this event. And I'm sure by Sunday we'll have all sorts of great statistics about how Hoquiam went from 86 one hour to 68 a few hours later.

But since we're still in the "waiting for it to happen" mode, I thought I'd take some time to explain just how heat waves work around here, and the complex blueprint that we'll be wading through over the next couple of days. First, let's talk about thermal troughs, which are also known as heat lows. Normally, low pressure is associated with bad weather, but in this case, low pressure is also connected to very hot weather. That's because hot air rises, and when you get a lot of hot air, you get a lot of rising air. That creates lower pressure at the ground due to this "missing" air, and thus where you get the term "heat low" ("Thermal trough" just sounds more scientific and, therefore, like we know what we're doing. Just like doctors say "a contusion on the upper right thigh" just means you've got a bruise.)

Mother Nature is not happy with air imbalances, which is why we have wind -- air flows from higher pressure to lower pressure. So air will flow toward the center of a thermal trough.

Now, most of the time, our predominant wind flow has some sort of westerly component -- either northwest, due west, or southwest. That brings air off the nearly constant 50-55 degree Pacific Ocean and provides what we call our "natural air conditioning".

For us to get hot, we have to hold that ocean air back and get a wind with an easterly component. That is difficult to do -- in essence, the wind has to go backwards from the flow of the planet. Which is why heat waves are pretty rare here.

So we have to make sure A) the west-blowing jet stream is nowhere near the neighborhood and B) we have some sort of trigger to draw that air from the east.

Enter the thermal trough, where as long as it is to our west, it will draw air from the east. And for the Puget Sound/I-5 corridor area and North and Central coast, this is a double whammy thanks to the Cascades and Olympic Mountains to the east, respectively.

As that wind comes from the east and over the mountains, it will sink down the western slopes. As air sinks, it goes through a process called "adiabatic warming" -- or, it just gets warmer and drier. That will take what might have been an 80-degree parcel of air and turn it into 86 or 88. But the good news is, it also dries out the air (which, coming from Eastern Washington and southeastern B.C., was already dry to begin with), which is why when we got hot here, it is a dry, desert-like heat and not an oppressive St. Louis-type muggy mess.

That's the basic set up. Now let's put this weeks' thermal trough in motion:

So, how can we forecast them? You can find a thermal trough on a chart that shows expected sea-level pressure. Here is one for Friday at 5 a.m.:

The black lines represent sea-level pressure. Air flows from higher pressure to lower pressure. Note how there's higher pressure just north of Idaho (a 1028mb High Pressure center) and lower pressure as you head west -- thus, a northeast wind. As for finding the thermal trough -- see those "upside horseshoes"? Draw a line through the tip of each curve like this:

That line is the thermal trough, which is where the wind is heading. If you're on the west side of that line, you'll have a cool, ocean-borne west wind. If you're on the east side, you'll have a hot, east wind.

You can see why it's expected to be hot on Friday.

(You should see my actual hand drawings. My mom's a professional artist, and somehow, it completely skipped getting passed down to me.)

The forecasting challenge is now watching that line eventually drift east. Also, the trough tends to align itself a bit northwest-to-southeast. First, it'll usually pass the Oregon Coast, then move north along the Washington Coast, before then passing to the east over the Puget Sound area, and into Eastern Washington.

So starting Friday, we'll be able to watch with amusement, and probably a little jealousy, as the trough passes locations along the Oregon coast on its slow trek here.

When the trough passes a location, the wind shift from the east to the west is usually quite sudden, and so is the ensuing temperature drop. For example, it wouldn't surprise me to see Newport be, say, 88 at 3 p.m., and 65 at 6 p.m.

Meanwhile, we're all sizzling in the interior locations as that east wind keeps on blowing.

On Saturday, the trough should start passing up the Washington Coast, providing sudden relief there from the south. Still sizzling in Seattle though.

By Saturday night, the trough is passing through the Puget Sound region, on it's way to the east. (See:

That will allow some cooler, west in for Sunday, but now signs are that there may not be a big push of marine air just yet. So Sunday will be quite a bit cooler, but not the big 30-35 degree drop that we can sometimes see.

Instead, the big push looks to hold until Sunday night, meaning Monday is when we're all expected to be down into the 60s again as life returns to some semblance of normal. Could even see some showers then, so there will likely be some sort of blog entry titled "From Sizzle to Drizzle" :)

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